Electrochromic devices are of increasing importance particularly in the automotive industry and the invention will be described for specific use in such industry although a wider use clearly is apparent.
In the automotive industry, electrochromic devices would be useful in the glass portions of an automotive vehicle. More particularly, an electrochromic windshield would be useful so that the transmission of light into the passenger compartment could be regulated to facilitate the heat management of such compartment. For example, it is advantageous to control the transmittance of infrared and visible radiation from the sun into the passenger compartment during the heat of the day to reduce the load on the air conditioning requirement. Similarly, it would be desirable to reduce the transmittance of visible light through the side windows of an automobile to permit privacy to its interior when desired. As a consequence, the automotive industry is showing considerable interest in so-called "smart windows" through which light transmittance can be controlled either manually or automatically.
Additionally, there also are electrochromic applications, such as for a rear-view mirror of an automotive vehicle, in which it would be desirable to change its reflectance automatically in response to photoelectric sensing of approaching headlights, to reduce the glare on the driver of the vehicle.
Electrochromic devices typically involve a rigid passive substrate that provides structural support, such as a layer of glass, for the active portion which comprises a multilayer sandwich of elements. Generally, the two outer layers of the active portion serve as the two terminal electrodes for the electronic current of the device. Between these terminal electrodes are sandwiched an electrochemical cell which generally comprises in succession a layer of an electrochromic material, a layer of an electrolyte, either solid or liquid, and a counter electrode. When a dc voltage is applied across the two terminal electrodes, electrons are either injected in or ejected out of the electrochromic layer at its interface with the terminal electrode while suitable ions are injected into or ejected from the electrochromic layer at its electrolyte interface. The electrolyte must conduct the ions participating in the electrochemical reaction that causes the color change. The counter electrode must be reversible to the same ions since it must allow the color changing reaction to proceed in the reverse direction so that the electrochromic layer may be also uncolored or "bleached" when desired. The counter electrode may also store the ions involved in the coloration reaction. In some instances, the counter electrode may also serve as one of the two terminal electrodes. For most applications, the electrolyte, the counter electrode and the two terminal electrodes need to be transparent.
The depth of color change and the resulting transmittance change in the electrochromic layer depend on the amount of ionic charge injected into or ejected from the electrochromic layer. In one sense, an electrochromic cell is basically a battery with a visible state of charge.
A wide variety of materials are known to be useful for coloring in the manner described and include some which can be colored cathodically in an electrochemical cell, such as tungstic oxide (WO.sub.3) and molybdenum oxide (MoO.sub.3), and others which can be colored anodically in an electrochemical cell, such as iridium oxide.
Similarly, a wide range of materials have been proposed, both liquid and solid, for use as the electrolyte. Superionic conductors and polymeric ion conductors are available in solid form.
Typically, the terminal electrodes are of transparent tin oxide doped to be highly conductive.
When an electrochromic device of the kind described has been used as the window of an automobile, the window is often subject to non-uniform coloration (darkening) and bleaching (lightening) as its transmission is varied. Such non-uniformity is undesirable and the present invention provides a solution to this problem.